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<a name="boost.proto.protect"></a><div class="titlepage"></div>
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<h2><span class="refentrytitle">Struct template protect</span></h2>
<p>boost::proto::protect — A <a class="link" href="../../PrimitiveTransform.html" title="Concept PrimitiveTransform">PrimitiveTransform</a> which prevents another
          <a class="link" href="../../PrimitiveTransform.html" title="Concept PrimitiveTransform">PrimitiveTransform</a> from being applied in an
          <a class="link" href="../../ObjectTransform.html" title="Concept ObjectTransform">ObjectTransform</a>.</p>
</div>
<h2 class="refsynopsisdiv-title">Synopsis</h2>
<div class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: &lt;<a class="link" href="../../proto/reference.html#header.boost.proto.transform.make_hpp" title="Header &lt;boost/proto/transform/make.hpp&gt;">boost/proto/transform/make.hpp</a>&gt;

</span><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <a class="link" href="../../PrimitiveTransform.html" title="Concept PrimitiveTransform">PrimitiveTransform</a><span class="special">&gt;</span> 
<span class="keyword">struct</span> <a class="link" href="protect.html" title="Struct template protect">protect</a> <span class="special">:</span> <span class="keyword"></span> <a class="link" href="transform.html" title="Struct template transform">proto::transform</a><span class="special">&lt;</span> <span class="identifier">protect</span><span class="special">&lt;</span><span class="identifier">PrimitiveTransform</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">{</span>
  <span class="comment">// member classes/structs/unions</span>
  <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span><span class="special">,</span> <span class="keyword">typename</span><span class="special">,</span> <span class="keyword">typename</span><span class="special">&gt;</span> 
  <span class="keyword">struct</span> <a class="link" href="protect/impl.html" title="Struct template impl">impl</a> <span class="special">{</span>
    <span class="comment">// types</span>
    <span class="keyword">typedef</span> <span class="identifier">PrimitiveTransform</span> <a class="link" href="protect/impl.html#boost.proto.protect.impl.result_type"><span class="identifier">result_type</span></a><span class="special">;</span>
  <span class="special">}</span><span class="special">;</span>
<span class="special">}</span><span class="special">;</span></pre></div>
<div class="refsect1">
<a name="id-1.3.31.5.52.5.4"></a><h2>Description</h2>
<p>
            When building higher order transforms with
            <code class="computeroutput">
              <a class="link" href="make.html" title="Struct template make">proto::make&lt;&gt;</a>
            </code> or
            <code class="computeroutput">
              <a class="link" href="lazy.html" title="Struct template lazy">proto::lazy&lt;&gt;</a>
            </code>,
            you sometimes would like to build types that are parameterized with Proto transforms. In such
            lambda-style transforms, Proto will unhelpfully find all nested transforms and apply them, even
            if you don't want them to be applied. Consider the following transform, which will replace the
            <code class="computeroutput">proto::_</code> in
            <code class="computeroutput">Bar&lt;proto::_&gt;()</code>
            with <code class="computeroutput">proto::terminal&lt;int&gt;::type</code>:
          </p>
<p>
            </p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">&gt;</span>
<span class="keyword">struct</span> <span class="identifier">Bar</span>
<span class="special">{</span><span class="special">}</span><span class="special">;</span>

<span class="keyword">struct</span> <span class="identifier">Foo</span> <span class="special">:</span>
  <a class="link" href="when.html" title="Struct template when">proto::when</a><span class="special">&lt;</span><a class="link" href="_.html" title="Struct _">proto::_</a><span class="special">,</span> <span class="identifier">Bar</span><span class="special">&lt;</span><a class="link" href="_.html" title="Struct _">proto::_</a><span class="special">&gt;</span><span class="special">(</span><span class="special">)</span> <span class="special">&gt;</span>
<span class="special">{</span><span class="special">}</span><span class="special">;</span>

<a class="link" href="terminal.html" title="Struct template terminal">proto::terminal</a><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span><span class="special">::</span><span class="identifier">type</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">{</span><span class="number">0</span><span class="special">}</span><span class="special">;</span>

<span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="special">)</span> <span class="special">{</span>
  <span class="identifier">Foo</span><span class="special">(</span><span class="special">)</span><span class="special">(</span><span class="identifier">i</span><span class="special">)</span><span class="special">;</span>
  <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="keyword">typeid</span><span class="special">(</span><span class="identifier">Foo</span><span class="special">(</span><span class="special">)</span><span class="special">(</span><span class="identifier">i</span><span class="special">)</span><span class="special">)</span><span class="special">.</span><span class="identifier">name</span><span class="special">(</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span></pre>
<p>
          </p>
<p>
            If you actually wanted to default-construct an object of type
            <code class="computeroutput">Bar&lt;proto::_&gt;</code>, you would have to protect the
            <code class="computeroutput">_</code> to prevent it from being applied. You can
            use <code class="computeroutput">proto::protect&lt;&gt;</code> as follows:
          </p>
<p>
            </p>
<pre class="programlisting"><span class="comment">// OK: replace anything with Bar&lt;_&gt;()</span>
<span class="keyword">struct</span> <span class="identifier">Foo</span> <span class="special">:</span>
  <a class="link" href="when.html" title="Struct template when">proto::when</a><span class="special">&lt;</span><a class="link" href="_.html" title="Struct _">proto::_</a><span class="special">,</span> <span class="identifier">Bar</span><span class="special">&lt;</span><a class="link" href="protect.html" title="Struct template protect">proto::protect</a><span class="special">&lt;</span><a class="link" href="_.html" title="Struct _">proto::_</a><span class="special">&gt;</span> <span class="special">&gt;</span><span class="special">(</span><span class="special">)</span> <span class="special">&gt;</span>
<span class="special">{</span><span class="special">}</span><span class="special">;</span></pre>
<p>
          </p>
</div>
</div>
<div class="copyright-footer">Copyright © 2008 Eric Niebler<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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